This is #2 of 2 bugs that I consider “urgent” in that the longer they are not fixed, the more “entropy” they will generate which will be increasingly hard to remove once the features will work again.
If you enter a new username/password combination in the authentication fields of a web page, OmniWeb offers no way of saving them in the macOS keychains.
Earlier on, a possible workaround would have been to once use Safari for entering a new username/password pair. But now, unfortunately this does not work anymore since Safari now writes the data to a keychain that OmniWeb does not read.
So, with the continued usage of OmniWeb, more and more new username/password combinations will get lost or be temporarily saved somewhere where you won’t find them afterwards …